In systems having rotating drive shafts it is sometimes necessary to know the torque and speed of these shafts in order to control the same or other devices associated with the rotatable shafts. Accordingly, it is desirable to sense and measure the torque applied to these items in an accurate, reliable and inexpensive manner.
Sensors to measure the torque imposed on rotating shafts, such as but not limited to shafts in vehicles, are used in many applications. For example, it might be desirable to measure the torque on rotating shafts in a vehicle's transmission, or in a vehicle's engine (e.g., the crankshaft), or in a vehicle's automatic braking system (ABS) for a variety of purposes known in the art.
One application of this type of torque measurement is in electric power steering systems wherein an electric motor is driven in response to the operation and/or manipulation of a vehicle steering wheel. The system then interprets the amount of torque or rotation applied to the steering wheel and its attached shaft in order to translate the information into an appropriate command for an operating means of the steerable wheels of the vehicle.
Prior methods for obtaining torque measurement in such systems was accomplished through the use of contact-type sensors directly attached to the shaft being rotated. For example, one such type of sensor is a “strain gauge” type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and the applied torque is measured by detecting a change in resistance, which is caused by applied strain and is measured by a bridge circuit or other well-known means.
Another type of sensor used is a non-contact torque sensor wherein magnetostrictive materials are disposed on rotating shafts and sensors are positioned to detect the presence of an external flux which is the result of a torque being applied to the magnetostrictive material.
Such magnetostrictive materials require an internal magnetic field which is typically produced or provided by either pre-stressing the magnetostrictive material by using applied forces (e.g., compressive or tensile) in either a clockwise or counter clockwise to pre-stress the coating prior to magnetization of the prestressed coating in order to provide the desired magnetic field. Alternatively, an external magnet or magnets are provided to produce the same or a similar result to the magnetostrictive material.
To this end, magnetostrictive torque sensors have been provided wherein a sensor is positioned in a surrounding relationship with a rotating shaft, with an air gap being established between the sensor and shaft to allow the shaft to rotate without rubbing against the sensor. A magnetic field is generated in the sensor by passing electric current through an excitation coil of the sensor. This magnetic field permeates the shaft and returns back to a pick-up coil of the sensor.
The output of the pick-up coil is an electrical signal that depends on the total magnetic reluctance in the above-described loop. Part of the total magnetic reluctance is established by the air gap, and part is established by the shaft itself, with the magnetic reluctance of the shaft changing as a function of torque on the shaft. Thus, changes in the output of the pick-up coil can be correlated to the torque experienced by the shaft.
As understood herein, the air gap, heretofore necessary to permit relative motion between the shaft and sensor, nonetheless undesirably reduces the sensitivity of conventional magnetostrictive torque sensors. As further understood herein, it is possible to eliminate the air gap between a shaft and a magnetostrictive torque sensor, thereby increasing the sensitivity of the sensor vis-a-vis conventional sensors. Moreover, the present disclosure recognizes that a phenomenon known in the art as “shaft run-out” can adversely effect conventional magnetostrictive torque sensors, and that a system can be provided that is relatively immune to the effects of shaft run-out.